The disclosed embodiments generally relate to the fields of optical networks, data switching and data routing.
In recent history, telecommunication systems and data networking systems have rapidly grown in size, speed and capacity. Accompanying the growth of these systems, however, has been the cost of maintaining these systems. A typically local area network (LAN) requires a large and costly infrastructure. For example, groups of servers must be included in the LAN to handle requests from users of the LAN, direct these requests accordingly, maintain various shared files and other resources, and provide a gateway to other networks, e.g., the Internet. In addition to the servers, each LAN must have a series of routers and switches to direct traffic generated by the users of the LAN. The servers, switches and routers, as well as the user's computers must all be connected via cables or wireless connections. These various devices and connections all require significant power, cooling, space and financial resources to ensure proper functionality.
Recently, fiber optic cables have been used to replace standard coaxial or copper based connections in communication networks. Fiber optic cables typically use glass or plastic optical fibers to propagate light through a network. Specialized transmitters and receivers utilize the propagated light to send data through the fiber optic cables from one device to another. Fiber optic cables are especially advantageous for long-distance communications, because light propagates through the fibers with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters, thereby reducing the costs of the communication networks.
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology that multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths of light to carry different signals. WDM allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber.
A WDM system typically uses a multiplexer to join multiple optical carrier signals together at a transmitter, and a demultiplexer at the receiver to split the multiplexed signal into its original optical carrier signals. WDM systems are generally broken into three different wavelength patterns: conventional, coarse and dense.
Dense Wavelength Division Multiplexing (“DWDM”) refers to optical signals multiplexed within the 1550-nm band. Conventional networks employing DWDM have operated between about 1530 nm and about 1565 nm because conventional fiber optic cables exhibited the best characteristics in this range. More recently developed fiber optic cables, such as Corning's SMF 28e, which is designed for use in metropolitan area networks (MANs), are able to support wavelengths ranging from about 1260 nm to about 1625 nm (about 238,095 GHz to about 184,615 GHz).
Similar to the above discussed WDM system, a DWDM system typically includes several components. First, a DWDM terminal multiplexer is typically used to receive any input carrier signals, convert the carrier signals to an appropriate wavelength, and multiplex the signals into a single multiplexed signal for transmission. Another component, depending on the size of the DWDM system, is an intermediate optical terminal. An intermediate optical terminal is used to remotely amplify the multiplexed signal. Typically, at about 140 kilometers, fiber optic signal quality begins to diminish, and an optical terminal may be used to strengthen the signal. A third typical component is a DWDM terminal demultiplexer. The DWDM terminal demultiplexer breaks the multiplexed signal back into individual signals at a receiver, and outputs the various individual signals on separate fibers for client-level systems.
Table 1 provides a list of band designations specified by the International Telecommunication Union for the main transmission regions of fiber optic cables and the wavelength ranges covered by each transmission region. Typically, DWDM falls into the 1530-1565 nm range, however, as mentioned above, advances in materials and construction methods for optical fibers has increased this range to nearly the entire range of main transmission regions, i.e., 1260-1625 nm.
TABLE 1ITU Standard Optical Band DefinitionsBandDescriptorWavelength RangeO bandOriginal1260-1360 nmE bandExtended1360-1460 nmS bandShort Wavelength1460-1530 nmC bandConventional1530-1565 nmL bandLong Wavelength1565-1625 nmU bandUltralong Wavelength1625-1675 nm
Conventional optical modulation schemes are based on Non-Return-to-Zero (NRZ) algorithms, which deliver 1 bit per Hz used. In a modulation scheme based on an NRZ algorithm, the value one is represented by a first significant condition (e.g., a positive voltage or light on), and a zero is represented by a second significant condition (e.g., a negative voltage or light off). Because such a modulation scheme has no rest or neutral position between bits, the bandwidth used is significantly reduced. However, NRZ algorithms are not inherently self-synchronizing. Synchronization errors may occur during a long string of consecutive values (e.g., a long string of ones or zeroes). An additional synchronization technique (such as a run length limited constraint or a parallel synchronization signal) can be used to avoid any potential bit slip or other synchronization errors.
Current optical systems can also employ more complex coding schemes, such as Duo binary (commonly used for 40 Gbps optical DWDM systems), which encodes 2 bits/Hz. In Duo binary modulation schemes, the value one is represented by two separate significant conditions (e.g., both a positive and a negative voltage), and the value zero is represented by a third significant condition (e.g., a zero net voltage). The Duo binary encoding scheme provides for a rapid fluctuation in voltage, thereby reducing the build-up of any direct current in a transmission signal, allowing the transmission signal to be transmitted over longer distances without signal degradation.
As communication systems grow and fiber optic systems become more integrated into standard communications, the speed and resultant cost of individual network components is also growing. Huge investments must be made by telecommunication companies to keep up with consumer demand as well as technological developments. As a result, telecommunication companies, as well as business running their own communication networks, would benefit greatly from network components with reduced space, weight, cost and power requirements. However, development has progressed slowly in this area. Instead, network components have become bigger and heavier and consume more power in the pursuit of supplying higher bandwidth.
In atypical environments, such as airborne or shipborne networks, space, weight and power become even more important for network design. However, the lack of progress in reducing the space, weight and power of network components described above has restricted the availability of high-bandwidth networks in such environments.
For example, space is at a premium on most airplanes and smaller ships. As such, network components of the size used in most business environments could exceed the available storage space in aerospace or naval environments. Data networks capable of providing on-demand video and audio programming to airplane passengers have developed slowly at least because of the size of conventional networking equipment. Similarly, military aircraft often require high-speed communication between subsystems or are used as a flying communication hub. However, conventional networking equipment is limited in its ability to perform this task because of the limited footprint that can be provided to all functions in an aircraft.
In addition, the weight of a network component has a direct effect on fuel consumption in airborne or shipborne environments because the added weight increases the drag on the airplane or ship. Similarly, the amount of power consumed by network components directly affects fuel consumption since power in airborne and shipborne environments is generated within the environment itself. For ships that are at sea for long periods of time, the power consumed by conventional networking equipment inhibits the ability to use such equipment because of the drain on limited energy reserves.